FIG. 1 is a configuration diagram of a magnetron drive electric source using a boosting transformer as a subject of the invention. In FIG. 1, an alternating current from a commercial electric source 11 is rectified into a direct current by a rectifier circuit 13. The direct current is smoothed by a choke coil 14 and a filter capacitor 15 on an output side of the rectifier circuit 13 and then given to an input side of an inverter 16. The direct current is converted into a desired high frequency (20 to 40 kHz) by the ON/OFF action of a semiconductor switching device in the inverter 16. The inverter 16 has two groups of switching devices, and a drive circuit for driving the two groups of switching devices. For example, each of the switching device groups is constituted by a plurality of power MOSFETs connected in parallel with one another to perform high-speed switching of the direct current. Drains of the power MOSFETs constituting the switching device groups are connected to opposite ends of a primary winding 181 of the boosting transformer 18. Sources of the power MOSFETs constituting one of the two switching device groups are connected to sources of the power MOSFETs constituting the other switching device group respectively. Gates of the power MOSFETs constituting the switching device groups are connected to the switching device drive circuit. The switching device groups constituted by the power MOSFETs are driven by an inverter control circuit 161 so that a current flowing in the primary side of the boosting transformer 18 is switched ON/OFF at a high speed.
A current on the primary side of the rectifier circuit 13 is detected by a CT 17. The detected current is input into the inverter control circuit 161 and used as an input signal for controlling the inverter 16.
A high-frequency voltage output from the inverter 16 is supplied to the primary winding 181 of the boosting transformer 18, so that a high voltage in proportion to the turn ratio in the boosting transformer 18 is obtained in a secondary winding 182 of the boosting transformer 18. A winding 183 having a small number of turns is further provided on the secondary side of the boosting transformer 18. The winding 183 is used for heating a filament 121 of a magnetron 12. The secondary winding 182 of the boosting transformer 18 is provided with a voltage doubler half-wave rectifier circuit 19 for rectifying the output of the secondary winding 182. The voltage doubler half-wave rectifier circuit 19 has a high-voltage capacitor 191, and two high-voltage diodes 192 and 193. In a positive cycle (e.g., on the assumption that the upper end of the secondary winding 182 in FIG. 1 is positive), a current flows in the high-voltage capacitor 191 and the high-voltage diode 192 so that left and right electrodes of the high-voltage capacitor 191 in FIG. 1 are charged positively and negatively respectively. Then, in a negative cycle (e.g., on the assumption that the lower end of the secondary winding 182 in FIG. 1 is positive), a current flows in the high-voltage diode 193 so that a doubled voltage which is the sum of a voltage from the high-voltage capacitor 191 previously charged and a voltage from the secondary winding 182 is applied between an anode 122 and a cathode 121 in the magnetron 12.
Although an example of the magnetron drive electric source using the boosting transformer as a subject of the invention has been described above, the drive electric source is not limited thereto. Any drive electric source may be used if it includes a transformer for boosting a high frequency.
With the needs of reduction in size of a microwave oven, it is necessary to reduce the size of a boosting transformer. Therefore, a low frequency heretofore used has begun to be replaced by a high frequency as described above. In a low frequency, a metal core (of an amorphous or silicon steel plate) advantageous in reduction in size, saturation and cost was used as a core in the transformer. In a high frequency, such a metal core has not been used because of large high-frequency loss and has begun to be replaced by a ferrite core.
FIG. 7 shows an example of the boosting transformer using ferrite cores. In FIG. 7, a primary winding 71, a secondary winding 72 and a heater winding 73 are disposed in parallel with one another on a common axis of two U-shaped ferrite cores 74 and 75 opposite to each other. In the case of a magnetron drive electric source often used for large electric power, a zero-volt switching method (hereinafter referred to as “ZVS method”) using voltage resonance is prevailingly used for lightening the load imposed on power semiconductor. A gap G is provided because the coupling coefficient of the boosting transformer needs to be selected to be in a range of from about 0.6 to about 0.85 in order to obtain a resonance voltage by the ZVS method.
In the case of the related-art boosting transformer using two U-shaped ferrite cores 74 and 75 opposite to each other, however, the peak current flowing in the primary side of the boosting transformer needs to be increased more greatly in order to make the output of the magnetron higher. As a result, magnetic flux density is saturated easily because the ferrite cores are inferior in saturation magnetic flux density characteristic. Therefore, increase in size of the ferrite cores is required in order to prevent saturation. This becomes a barrier to the major premise that the size of the electric source needs to be reduced.